The redox reaction of cytochrome c at a gold electrode in the presence of 4,4'-bipyridyl (4-bpy) as a surface modifier was investigated spectroelectrochemically in a neutral aqueous solution. Two types of the redox processes of cytochrome c were studied in detail. One (process I) is the diffusion-controlled reaction of cytochrome c from the solution phase to the electrode surface covered by the adsorbed layer of 4-bpy, at the same formal potential as the native cytochrome c. The other (process II) involves both the reaction of cytochrome c coadsorbed with 4-bpy on the gold electrode surface and the mediated reduction of cytochrome c in the solution phase. It was found that the formal potential of cytochrome c coadsorbed with 4-bpy became more positive as the concentration of 4-bpy increased. This is probably due to the fact that the coadsorbed 4-bpy acts to reduce the degree of unfolding of adsorbed cytochrome c. Electroreflectance spectra of each of the two processes were very different. It was found that the electronic structure of the cytochrome c coadsorbed with 4-bpy was different from that of the native cytochrome c.

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